Implantable cardiac stimulation devices are well known in the art. They include implantable pacemakers which provide stimulation pulses to cause a heart, which would normally beat too slowly or at an irregular rate, to beat at a controlled normal rate. They also include defibrillators which detect when the atria and/or the ventricles of the heart are in fibrillation or a pathologic rapid organized rhythm and apply cardioverting or defibrillating electrical energy to the heart to restore the heart to a normal rhythm. Implantable cardiac stimulation devices may also include the combined functions of a pacemaker and a defibrillator.
As is well known, implantable cardiac stimulation devices sense cardiac activity for monitoring the cardiac condition of the patient in which the device is implanted. By sensing the cardiac activity of the patient, the device is able to provide cardiac stimulation pulses when they are needed and inhibit the delivery of cardiac stimulation pulses at other times. This inhibition accomplishes two primary functions. Firstly, when the heart is intrinsically stimulated, its hemodynamics are generally improved. Secondly, inhibiting the delivery of a cardiac stimulation pulse reduces the battery current drain on that cycle and extends the life of the battery which powers and is located within the implantable cardiac stimulation device. Extending the battery life, will therefore delay the need to explant and replace the cardiac stimulation device due to an expended battery. Generally, the circuitry used in implantable cardiac stimulation devices have been significantly improved since their introduction such that the major limitation of the battery life is primarily the number and amplitude of the pulses being delivered to a patient's heart. Accordingly, it is preferable to minimize the number of pulses delivered by using this inhibition function and to minimize the amplitude of the pulses where this is clinically appropriate.
It is well known that the amplitude value of a pulse that will reliably stimulate a patient's heart, i.e., its threshold value, will change over time after implantation and will vary with the patient's activity level and other physiological factors. To accommodate for these changes, pacemakers may be programmed to deliver a pulse at an amplitude well above (by an increment or a factor) an observed threshold value. To avoid wasting battery energy, the capability was developed to automatically adjust the pulse amplitude to accommodate for these long and short term physiological changes. In an existing device, the Affinity™ DR, Model 5330 L/R Dual-Chamber Pulse Generator, manufactured by the assignee of the present invention, an AutoCapture™ pacing system is provided. The User's Manual, ©1998 St. Jude Medical, which describes this capability is incorporated herein by reference. In this system, the threshold level is automatically determined in a threshold search routine and is maintained by a capture verification routine. Once the threshold search routine has determined a pulse amplitude that will reliably stimulate, i.e., capture, the patient's heart, the capture verification routine monitors signals from the patient's heart to identify pulses that do not stimulate the patient's heart (indicating a loss-of-capture). Should a loss-of-capture (LOC) occur, the capture verification routine will generate a large amplitude (e.g., 4.5 volt) backup pulse shortly after (typically within 80–100 ms) the original (primary) stimulation pulse. This capture verification occurs on a pulse-by-pulse basis and thus, the patient's heart will not miss a beat. However, while capture verification ensures the patient's safety, the delivery of two stimulation pulses (with the second stimulation pulse typically being much larger in amplitude) is potentially wasteful of a limited resource, the battery capacity. To avoid this condition, the existing device, monitors for two consecutive loss-of-capture events and only increases the amplitude of the primary stimulation pulse should two consecutive loss-of-capture (LOC) events occur. This procedure is repeated, if necessary, until two consecutive pulses are captured, at which time a threshold search routine will occur. The threshold search routine decreases the primary pulse amplitude until capture is lost on two consecutive pulses and then, in a similar manner to that previously described, increases the pulse amplitude until two consecutive captures are detected. This is defined as the capture threshold. The primary pulse amplitude is then increased by a working margin value to ensure a primary pulse whose amplitude will exceed the threshold value and thus reliably capture the patient's heart without the need for frequent backup pulses.
While the method used by this existing device has been proven to be a safe and effective method of determining a primary pulse amplitude that will reliably capture the patient's heart, conditions have been encountered that may cause the prior art system to rely upon backup pulses for extended periods of time, thus expending battery capacity to ensure patient safety. For example, in one case, a pulse amplitude may be set just below the threshold of a patient's heart (see the first portion of FIG. 8). Due to various factors, e.g., electrical noise, myopotentials or the like, the prior art system may alternate between capture and loss-of-capture events for an extended period of time. In a next case (see FIG. 9), a heart may be subject to periods of PVCs (premature ventricular contractions) generated by ectopic foci in the heart. Following a PVC, the heart muscle may be subject to supernormal conduction due to a longer repolarization period. Accordingly, an otherwise subthreshold primary pulse may still be capable of causing capture of the patient's heart for the next cardiac cycle. Accordingly, a trigeminy type pattern that alternates between loss-of-capture, a PVC (appearing as a sensed event), and a captured pulse may repeatedly occur. Since such a pattern does not satisfy the two sequential loss-of-capture event requirement of the existing device, the existing device will not be able to adapt to this condition. Although, patient safety is maintained, it is done at the expense of battery life.
Therefore what is needed is a flexible system that can determine a threshold amplitude for the primary pulse in such special cases as well as normal cases and that can preferably minimize the time to determine the threshold amplitude.